Problem: The lifespans of snakes in a particular zoo are normally distributed. The average snake lives $25.3$ years; the standard deviation is $2.7$ years. Use the empirical rule (68-95-99.7%) to estimate the probability of a snake living between $17.2$ and $19.9$ years.
Explanation: $25.3$ $22.6$ $28$ $19.9$ $30.7$ $17.2$ $33.4$ $99.7\%$ $95\%$ $2.35\%$ $2.35\%$ We know the lifespans are normally distributed with an average lifespan of $25.3$ years. We know the standard deviation is $2.7$ years, so one standard deviation below the mean is $22.6$ years and one standard deviation above the mean is $28$ years. Two standard deviations below the mean is $19.9$ years and two standard deviations above the mean is $30.7$ years. Three standard deviations below the mean is $17.2$ years and three standard deviations above the mean is $33.4$ years. We are interested in the probability of a snake living between $17.2$ and $19.9$ years. The empirical rule (or the 68-95-99.7 rule) tells us that $99.7\%$ of the snakes will have lifespans within 3 standard deviations of the average lifespan. It also tells us that $95\%$ of the snakes will have lifespans within 2 standard deviations of the mean. That leaves $99.7\% - 95\% = 4.7\%$ of snakes between 2 and 3 standard deviations of the mean, or $2.35\%$ on either side of the distribution. The probability of a particular snake living between $17.2$ and $19.9$ years is $\color{orange}{2.35\%}$.